Hi Cleo59,
just tried it with my Audya, and it works fine. I do not often use it, so it was not possible for me to answer faster.
I have the SW 5.0A
What I did to activate it:
Manual Bass must be activated (so that you can play it independent from the arranger)
In the Settings:
Menu/Keyboard Control/Accordeon
The Accordeon mode must be activated "Active"
With this I can change the sustain, octave etc.
The Bass sound itself depends on the current style selected.
Hope this helps
Joe